I have played this scenario a few times, and normally manage to win on emperor level. From my experience, the easiest way to win as France is to ignore the English home land, and attack one enemy at the time (in order: Netherlands, Italy, Spain, Portugal, Gibraltar and Prussia).
I have tried the England home island invasion just for fun (and with the idea to get out of war weariness after eliminating everyone in the locked alliance). It was very possible to do after getting the Grand Batteries. By taking London early on you interrupt the annoying free frigate every six turn bonus, and also take the Smith´s Trading Company. After taking a few of the other coastal cities (with harbors), it is actually possible to win the war at sea, at least with Ship of the line. This will take a lot of effort, slowing down the attack in Italy considerably, and making it very tempting for your friends in Austria and Prussia to stab you in the back. Also remember not to add the fourth unit to your armies, otherwise you wont be able to ship them back to the continent (troop transports can only carry an army with the size of three)!
On emperor level, I won using this fun but less effective strategy on score (but not reaching 60 000 pts), on monarch I managed to win at 60 0000 by eliminating Netherlands, England, Portugal, Italy, Prussia and attacking Russia (just for fun).
I am interested to hear if someone managed to win on Deity or higher, and what strategies you used.
/En passant